AN1071 Datasheet, PDF (1/6 Pages) List of Unclassifed Manufacturers – Information About the LM2650 Evaluation Board

Information About the
LM2650 Evaluation Board
Rev. 1
National Semiconductor
AN-1071
Steven Hunt
September 1997
INTRODUCTION
A printed circuit board (PCB) has been developed. This ap-
plication note contains information about the board.
GENERAL DESCRIPTION
The LM2650 evaluation board is provided as a tool for devel-
oping DC/DC converters based on the LM2650 IC. It is con-
figured for single-output, step-down DC/DC converters. Fig-
ure 1 is a complete schematic of the board which can
accommodate up to 28 components including the LM2650.
Table 1 is a complete list of pads for placing components.
Note: Not all applications will require the placement of all 28
components. The number of components placed depends
on the requirements of the application and the use of fea-
tures like programmable soft-start. The LM2650 evaluation
board is intended to be a reusable tool on which many differ-
ent converters meeting the requirements of many different
applications can be built. It is not intended to demonstrate
only one application of the LM2650.
For convenience, a sample of the LM2650 and eight other
components have been assembled: a 0.1 µF capacitor at
each of C1, C2, C6, and CB, a 0.2 µF capacitor at CVDD, a
0.01 µF capacitor at CSS, a 24.9 kΩ resistor at R2, and a 0Ω
resistor at R4. Of the eight, the first seven are common to
many applications, and the last is simply a jumper grounding
the SYNC input. When the synchronization feature is not be-
ing used, the SYNC input should be grounded.
EXAMPLE CIRCUITS
The components contained in Tables 2, 3, and 4 can be used
to build typical application circuits. As with the design of any
DC/DC converter, the design of these involved tradeoffs be-
tween efficiency, size, and cost. The converters detailed in
Table 2 were designed with efficiency as the number one cri-
teria. Those detailed in Table 4 trade slightly higher switching
losses for a much smaller inductor.
FIGURE 1. The LM2650 Evaluation Board Rev. 1 Schematic
AN100011-5
THERMAL PERFORMANCE
The 24-lead SO package is a molded plastic package with a
solid copper lead frame. Most of the heat generated at the
die flows through the lead frame into the 3-ounce copper
planes on the board. The board then acts as a heat sink. The
junction-to-ambient thermal resistance of the packaged IC
mounted on the board has been measured to be 38˚C/W, 37
˚C/W, and 35 ˚C/W for the dissipation of 1.0W, 1.5W, and
2.0W respectively. These measurements were made in still
air. The junction-to-ambient thermal resistance of the pack-
aged IC alone in still air is 78˚C/W. The board is 0.063″ thick
FR-4 material.
ART
Figure 2 through Figure 4 show the PCB art work.
© 1997 National Semiconductor Corporation AN100011
www.national.com